Stack refers to a bow becoming suddenly much harder to pull. Bows build fairly consisstent weight through their power curve. WHen you get past a certain string angle, the pressure you have to put on the string to get another inch of draw increases dramatically. Feels like you've hit a wall.
I had an old Bear takedown (the cheap hunter, not one of the good ones) that required an additional 28lbs of pull to go from 28 inches of draw to 30 inches of draw. One of the worst stacking bows I've ever shot.